SEED PLANTS 175 



Cycads and Conifers, but on the whole they are prob- 

 ably nearer the latter. 



Summary 



Compared with the Angiosperms, the Gymnosperms 

 are an ancient primitive group of plants showing very 

 evident resemblances to the Pteridophytes, from which 

 they have doubtless originated. It is not likely that the 

 existing Gymnosperms constitute a homogeneous class, 

 but it is more probable that they are remnants of at 

 least three lines of development. Of these the Cycads 

 show evident relationships to the ferns, this being 

 evinced by the character of the leaf and flowers, and 

 still more by the form of the spermatozoids. The geo- 

 logical record shows that the Cycads were once a much 

 more important group than at present. Perhaps related 

 to these is the genus Gingko, also a very old type, with 

 but a single living representative. 



The Conifers are a more recent type than the Cycads, 

 but still are older than the Angiosperms. In the gen- 

 eral habit of the sporophyte, especially the sporophylls, 

 they suggest a direct connection with the Lycopods, 

 and this is borne out by a study of the gametophyte, 

 which closely resembles that of Selaginella. The pres- 

 ence in the Carboniferous rocks of gigantic Lycopods 

 now extinct, suggests these as possible ancestors for the 

 existing Conifers. 



There is very little in common between the third 

 order, the Gnetacese, and the other Gymnosperms, and 

 it is questionable whether the theory that the Gnetaceas 



